The invention can in particular be applied in telephone service payment systems, such as in the system referred to the patent application filed by Alain BERNARD on the data of filing the present application and entitled "Telephone service payment system".
There already exist acoustic coupler modems able to transmit sounds via a small speaker. This speaker, similar to the microphone of a telephone handset, allows for the transmission of these sounds onto the telephone network. Another modem, situated at the other extremity of the link, interprets the data it receives and establishes a connection with, for example, a computer.
The CCITT (Comite Consultatif International Telegraphic et Telephonic=The International Telegraph and Telephone Consultative Committee) defined in its notice Q23 a set of tones corresponding to the twelve figures and signs of the telephone keypad and thus it is possible to transmit these figures and signs with the aid of such modems.
Thus, there are a large number of models of boxes resembling pocket calculators making it possible to store telephone numbers and transmit them onto the telephone network.
In another sphere of application, there are numbering devices making it possible to add to a digital signal a set of time-evolving pseudo-random data so that the initial digital signal is only able to be located by virtue of knowing the set of data and its time evolution.
Finally, the identification of a person calling a computer connected to the telephone network is currently effected by asking this person to type a code number on the keypad.
One example of this prior art is cited in the document U.S. Pat. No. 4,601,011 which describes a device including means able to form a digital message once the correct secret code has been typed on the keypad. This message changes over a certain period of time by virtue of a counter whose contents are increased by increments on each use. These contents partly determine the digital message sent onto the telephone line.
Although satisfactory in certain respects, the identification of a person by means of a code number does exhibit a certain number of drawbacks as the code may be stolen. The other devices are complex and take up a large amount space, especially when they require a keypad. In the best of cases, the spatial requirement is reduced to the size of a pocket calculator. Furthermore, they are ill-adapted to define a period of validity or, if one prefers, a time limitation date.